As the problem of environmental pollution becomes a major social issue, exhaust gas regulations for internal combustion engine vehicles using fossil fuels are becoming increasingly strict. Particularly, exhaust gas from diesel engine vehicles using diesel fuel, such as buses and trucks contains nitrogen oxide (NOx) which has been turned out as a cause material of acid rain and respiratory illnesses. European emission standards which are applied recently or will be gradually strengthened regulate emission standards of nitrogen oxide more strictly.
In order to cope with such emission standards, a method of reducing nitrogen oxide contained in exhaust gas of a vehicle diesel engine by applying a selective catalytic reduction (SCR) system to a vehicle has been used. The SCR system removes nitrogen oxide in the exhaust gas by reducing the nitrogen oxide to nitrogen and water using a urea solution as a catalyst. The SCR system is currently used mainly for large vehicles such as a truck. Recently, however, the SCR system is expanding its application range to passenger cars. That is, the SCR system is a selective reduction system that injects the urea solution into an exhaust pipe of a vehicle, converts the injected urea solution into ammonia through pyrolysis and hydrolysis, allows the converted ammonia to react with nitrogen oxide in exhaust gas to convert the nitrogen oxide into harmless components such as water and nitrogen, and requires a system for supplying a urea solution.
To this end, a urea solution tank is equipped with a urea solution pump module which sucks and feeds the urea solution contained in the urea solution tank, in which the fed urea solution is injected into an exhaust pipe through an injector.
However, since the urea solution contains a large amount of water, if the SCR system is not operated for a long time in a low temperature state such as the winter season, the urea solution contained in the urea solution tank may be frozen and even the pump portion of the urea solution pump module may be frozen. Therefore, a heater for thawing them is needed.
Therefore, according to the related art, as shown in FIG. 1, a heater 30 is disposed outside a filter 20 surrounding a pump 10 so that a urea solution frozen inwardly from the outside of the filter 20 is thawed. However, since the urea solution frozen in the filter in which the pump is disposed may not be melted until a large amount of urea solution frozen outside the filter is melted, the pump cannot be operated within a rapid time. Korean Patent Laid-Open Publication No. 10-2014-0020035 entitled “urea solution pump unit” is disclosed as the related art.